The broad objective of this proposal is to contribute to the characterization of circadian rhythms in plants. By applying what is known about the mechanism, regulation, and influence of the plant clock to mammalian models, progress will be made toward the understanding and treatment of human circadian disorders. In Arabidopsis, genetic analyses have identified components of the plant circadian system. However, the function and position of these factors relative to the clock is not clear. One such factor, LKP2, has been implicated to act in the light input pathway and in proteolysis. The goal of this proposal is to ascertain the molecular activities of LKP2 and to determine how these actions affect the operation of the circadian clock. The specific aims of this proposal are: 1) investigate the role of LKP2 as a light sensor by examining chromophore binding and light regulation of protein stability/activity, 2) assess the involvement of LKP2 in ubiquitin-mediated protein degradation by assaying the functionality of the LKP2 F box, and 3) identify interacting proteins and genes via biochemical purification and by performing a suppressor screen. The findings of this study will assist in revealing the function and regulation of LKP2 as well as to uncover additional components of the plant circadian clock.